1. Field of the Invention
This invention relates to a vapour phase hydrogenation process.
2. Related Art
Various hydrogenation processes have been described which utilise vapour phase conditions. In such processes a normally liquid starting material containing one or more unsaturated groups selected from ##STR1## is typically vaporised in a stream of hot hydrogen or hydrogen-containing gas in a proportion such that there is formed a vaporous mixture of gas and vaporised starting material that is above the dew point of the mixture. This vaporous mixture is then passed into a hydrogenation zone containing a charge of a granular heterogeneous hydrogenation catalyst, typically a reactor containing a bed or beds of hydrogenation catalyst or a plurality of such reactors connected in series.
Another type of hydrogenation process is hydrodesulphurisation in which a hydrocarbon feedstock is contacted with hydrogen in the presence of a hydrodesulphurisation catalyst in order to reduce its sulphur content.
Examples of hydrogenation processes include hydrogenation of aldehydes to alcohols, of unsaturated hydrocarbons to saturated hydrocarbons, of ketones to secondary alcohols, of nitriles to primary amines, of alkyl esters of aliphatic monocarboxylic acids to alkanols, and of dialkyl esters of aliphatic dicarboxylic acids to aliphatic diols. Typical hydrogenation conditions include use of a pressure of from about 1 bar to about 100 bar and of a temperature of from about 40.degree. C. to about 250.degree. C., depending upon the nature of the hydrogenation reaction and the activity of the selected hydrogenation catalyst.
Hydrogenation is normally an exothermic reaction. Thus the hydrogenation reactor or reactors may be operated adiabatically or isothermally with external or internal cooling. Adiabatic reactors are used where possible for preference since they are usually cheaper to construct and to operate than an isothermal reactor of shell and tube design.
In a typical vapour phase hydrogenation process it is normally preferred to operate so that the temperature of the vaporous mixture in contact with the catalyst is always at least about 5.degree. C., and even more preferably at least about 10.degree. C., above its dew point. It is desirable to prevent contact of liquid with the catalyst because, due to the exothermic nature of the hydrogenation reaction, damage to the catalyst may result from formation of hot spots on the surface of the catalyst, leading possibly to sintering (particularly in the case of copper-containing catalysts), or from disintegration of the catalyst pellets as a result of explosive vaporisation within the pores of the catalyst pellets. Sintering is a particular hazard in the case of copper-containing hydrogenation catalysts and may result in a significant reduction in the exposed reduced copper surface area and in a consequent reduction in catalyst activity. Disintegration of catalyst pellets results in production of catalyst "fines" which causes in turn an increase in pressure drop across the catalyst bed and hence an increase in the power required to force the vaporous mixture through the hydrogenation zone. Such an increased power requirement increases the running costs for the plant operator. In addition allowance has to be made at the design stage in selecting the size of the gas recycle compressor to ensure that it can handle any increased pressure drop likely to be encountered during operation of the plant over the life of a catalyst charge. This can add significantly to the capital cost of the plant.
Vapour phase hydrogenation of aldehydes has been proposed, for example, in U.S. Pat. No. 2,549,416, and in EP-A-0008767. These proposals utilise a reduced copper oxide-zinc oxide catalyst.
Hydrogenolysis of esters to alcohols in the vapour phase is proposed in WO-A-82/03854. Again, the catalyst proposed is a reduced copper oxide-zinc oxide catalyst.
Production of butane-1, 4-diol, qamma-butyrolactone, and tetrahydrofuran by vapour phase hydrogenation of a dialkyl ester of a C.sub.4 dicarboxylic acid, for example a dialkyl maleate, such as diethyl maleate, using, preferably, a reduced copper chromite catalyst, has been described in EP-A-0143634, in WO-A-86/03189, and in WO-A-86/07358.
In plants using more than one hydrogenation reactor in series, particularly those using adiabatic hydrogenation reactors connected in series, it has been recognised that it will normally be necessary to control the temperature rise across each catalyst bed in order to avoid hot spot formation and to obviate the risk of temperature runaways occurring, for example. In many cases it is disadvantageous to operate with too high an exit temperature from the catalyst bed because high exit temperatures often result in increased formation of by-products.
One way of controlling the temperature rise across a catalyst bed of an adiabatic reactor is to increase the amount of hydrogen in the circulating gas or to allow inert gases, such as nitrogen, to build up in the recirculating gas. In this way the extra gas acts as a heat sink to absorb the exothermic heat of reaction. However, the size of the gas conduits and the size of the gas recycle compressor must be increased to allow for the increased gas flow and the capital costs of the plant are increased.
Use of a plurality of adiabatic reactors in series with injection of cold shots of gas between reactors is another commonly adopted expedient. Inter-bed cooling is another method of limiting the temperature rise across an adiabatically operated catalyst bed.
Vaporisation of additional starting material in the reaction mixture exiting one reactor, prior to entry of the ensuing mixture to the next reactor of a plurality of reactors connected in series, is suggested in EP-A-0215563.
In most vapour phase hydrogenation reactions some deactivation of the catalyst is observed with passage of time in operation of a vapour phase hydrogenation plant. This loss of catalyst activity is often accompanied by an increase in pressure drop across the catalyst bed because of the production of catalyst "fines" due to disintegration of catalyst particles.
Many hydrogenation reactions require pre-reduction of the catalyst prior to commencing hydrogenation of the unsaturated organic starting material. For optimum catalyst activity care has to be taken to effect such pre-reduction of the catalyst under controlled conditions which are normally specified by the catalyst manufacturer. Such controlled pre-reduction conditions often involve use of gaseous space velocities in excess of those normally encountered during hydrogenation. Hence it is necessary to ensure at the design stage that the gas recycle compressor is large enough to provide the necessary high circulation rates required during such catalyst activation. For this reason it may be necessary to provide a gas recycle compressor that is larger, and hence is more expensive, than would be required during normal operation of the hydrogenation plant.